1. Field of the Invention
The present invention relates to a programmable control system including a plurality of programmable controllers (or programmable ladder controllers, hereinunder referred to as ("PC's") for automatically controlling various pieces of equipment.
2. Description of the Related Art
PC's are used for controlling various pieces of equipment such as a transfer machine and a PC having more functions and easy to use has recently been demanded for realizing factory automation.
As shown in FIG. 1, a conventional PC 10 includes a memory 12 for storing a sequence program and a control processor 14 for controlling a piece of equipment in accordance with the sequence program. The PC 10 further includes I/O portion (not shown) connected to an external piece of equipment, data memory (not shown), system memory (not shown), etc. FIG. 2 shows what is called a relay ladder for showing the order and the conditions of control with respect to the equipment being controlled. Equipment being controlled includes virtual equipment (dummy equipment) which exists only in a program.
A relay symbol 16 in the relay ladder diagram represents a piece of equipment being controlled An address 18 shown in the vicinity of the relay symbol 16 is a local I/O address which is only effective in the PC 10. The I/O address is expressed by an octal digit, a hexadecimal digit or the like and individually allotted to each piece of equipment.
The order and the conditions of control shown in the relay ladder diagram are programmed by a programming language such as a logic symbol language and a relay symbol language and stored in the memory 12 as a sequence program. To state this more concretely, in the sequence program, a command and an I/O address to each piece of equipment are entered, as shown in FIG. 1 The sequence program memory 12 therefore includes a region 12a for storing a command and a region 12b for storing an address.
A programmable control system for controlling a transfer machine and the like is composed of a plurality of PC's. The PC's control the pieces of equipment in parallel with each other Each of the PC's therefore has its own sequence program.
When the operator corrects or checks a sequence program which is stored in a PC, it is necessary to connect the programming tool to the specific PC in which the sequence program being corrected or checked is stored.
The operator must therefore constantly know which program is stored in which PC It is, however, very difficult to remember all the programs, so that it is not easy to correct or check a program. In addition, in the case of correcting one program after another program has been corrected, it is necessary to connect the programming tool to the respective PC's in series, thereby deteriorating the operability.
Furthermore, since the I/O address in the program which is displayed on the programming tool is expressed merely by a numeral, it is difficult for the operator to judge which equipment the I/O address indicates. This leads to not only the deterioration of the operability but also to an input error.